This invention relates to a magnetron, and more particularly to a magnetron of the type wherein permanent magnets are disposed within the anode cylinder.
In a prior art magnetron, permanent magnets are arranged on the outside of the anode cylinder and magnetic yokes are used to form magnetic circuits. In the magnetron of this type since the permanent magnets are positioned remote from the interaction space between the cathode and anode electrodes, the leakage flux is increased and the utilization factor of the magnetic field is only few percent. For this reason, it is necessary to increase the size of the permanent magnets and the yokes for the purpose of providing magnetic field of the desired strength. Accordingly, there are problems of decreasing the size and the cost of manufacturing.
Recently, it has been proposed to dispose the permanent magnets in the anode cylinder for the purpose of increasing the utilization factor of the magnetic field thereby decreasing the size and cost of manufacturing. Such construction is disposed for example in Japanese Pat. application No. 50156/1972 filed May 17, 1972 (Layed open specification No. 15358/1974 published Feb. 9, 1974) entitled "Magnetron".
FIG. 1 of the accompanying drawing shows a longitudinal section of one example of the magnetron of this type in which the anode comprises an anode cylinder 12 made of ferromagnetic material and a plurality of vanes 11 secured to the cylinder 12 for defining a plurality of cavities. A cathode electrode 13 is supported about the axis of the anode cylinder 12 by means of a cathode support 14 to form the interaction space between the cathode electrode 13 and the vanes 11. Upper vacuum seal wall 15 and lower vacuum seal wall 16 are provided for the opposite ends of the anode cylinder 12. An annular ring shaped first permanent magnet 17 is mounted on the inside of the upper vacuum seal wall 15 by a frusto conical shaped support 18 made of stainless steel, for example, and provided with a central opening, and a short cylindrical second permanent magnet 19 is secured to the inner side of the lower vacuum seal wall 16 by means of a bonding agent. These first and second permanent magnets are magnetized in the axial direction of the anode cylinder 12. The upper and lower vacuum seal walls 15 and 16 are respectively provided with a cathode input terminal 20 for the cathode electrode 13 and an output antenna 21 which is also used as the exhaust tube. The cathode input terminal 20 is supported by an insulating bushing 25 made of ceramic, for example, and filter casings 22 and 23 containing filters for preventing unwanted leakage radiation is also mounted on the insulating bushing 25. The output antenna 21 is supported by an insulating bushing 26 made of ceramic, for example. A plurality of heat radiating plates 31 are secured on the outer surface of the anode cylinder 12.
In this manner, by disposing the first and second permanent magnets 17 and 19 in an evacuated casing formed by the upper and lower vacuum sealing walls 15 and 16 it is possible to eliminate a large outside yoke and outside permanent magnets which are necessary for the prior art construction thereby greatly reducing the weight and the size of the magnetron tube. However, there are still following disadvantages. Firstly, since the first permanent magnet 17 on the side of the cathode input terminal 20 is annular and the second permanent magnet 19 on the side of the output antenna 21 is a short cylinder, and since both permanent magnets are magnetized in the axial direction of the anode cylinder 12 the magnetic flux flows through a magnetic path has shown by a. Thus, after leaving the first permanent magnet 17 the flux flows toward the second permanent magnet 19 while converging toward the center of the interaction space. Consequently, as shown by P in FIG. 2, the flux density curve in the interaction space will incline. In FIG. 2, the abscissa represents the position along the axis of the anode cylinder, whereas the ordinate the flux density. As can be noted from FIG. 2, the flux density decreases toward the upper portion of the interaction space thus resulting in a non-uniform flux distribution. This broadens abnormal oscillation region which is generally called a moding region, thereby causing a undesired oscillation at the time of high efficiency operations. Thus, where it is desired to provide a .pi. mode oscillation, for example, oscillations other than the .pi. mode would result. Where the outer diameter of the first permanent magnet 17 is increased by taking into consideration the fact that it is impossible to utilize strong magnetic field on the side of the first permanent magnet, leakage flux flowing through a magnetic path b shown in FIG. 1 increases thus short-circuiting the effective magnetic path a. Thus, mere increase of the size of the first permanent magnet does not result in any advantageous effect. For the reasons described above, in the magnetron of the type shown in FIG. 1 there are such disadvantages that the flux density in the interaction space is low so that it is difficult to obtain magnetrons capable of producing high outputs with small and compact design.